Ink jet ink, ink cartridge, and ink jet recording method

ABSTRACT

An ink jet ink contains a polyurethane polymer and a pigment. The polyurethane polymer has units derived from a polyisocyanate, a polyether polyol having no acid group, and a diol having an acid group. The pigment is dispersed using a polymer other than the polyurethane polymer. The molar ratio of the percentage of the urethane bond in the polyurethane polymer to the percentage of the urea bond in the polyurethane polymer is 85.0/15.0 or more and 100.0/0 or less.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an ink jet ink, an ink cartridgecontaining the ink jet ink, and an ink jet recording method.

2. Description of the Related Art

In some ink jet inks, a pigment is dispersed using a polymer. Also insuch polymer-dispersed pigment inks, there has recently been a demandfor high ink reliability (including ejection stability and ejectionaccuracy), image quality (including high optical density), and imagefastness (including scratch resistance and highlighter resistance). Inorder to improve these characteristics, various polymer-dispersedpigment inks containing a polyurethane polymer have been studied(Japanese Patent Laid-Open No. 2005-290044 and Japanese Patent Laid-OpenNo. 2008-266595). Japanese Patent Laid-Open No. 2005-290044 discloses anink that contains a pigment dispersed using a polyurethane polymer.Japanese Patent Laid-Open No. 2008-266595 discloses an ink that containsa polymer-dispersed pigment and a polyurethane polymer having a carboxygroup.

Inks containing a polyurethane polymer have been studied in which thenumber of urea bonds in the polyurethane polymer is specified inconsideration of the correlation between the urea bond and thecharacteristics of the ink (Japanese Patent Laid-Open No. 9-291242 andJapanese Patent Laid-Open No. 2004-285344). Japanese Patent Laid-OpenNo. 9-291242 discloses an aqueous printing ink that contains apolyurethane polymer having a specific ratio of the urethane bond to theurea bond. The aqueous printing ink has improved adhesion to a plasticfilm and improved laminate strength. Japanese Patent Laid-Open No.2004-285344 discloses an ink that contains an acrylic-polymer-dispersedpigment and a polyurethane polymer having no urea bond. The ink hasimproved storage stability and improved gloss.

The present inventors found that conventional pigment inks containing apolyurethane polymer have improved ink reliability and image fastnessbut still do not sufficiently satisfy the requirements.

The present inventors found that the use of a pigment dispersed using apolyurethane polymer as described in Japanese Patent Laid-Open No.2005-290044 results in poor ink ejection stability. This is because thepolyurethane polymer described in Japanese Patent Laid-Open No.2005-290044 is not hydrophilic enough to disperse the pigment. Thepolyurethane polymer described in Japanese Patent Laid-Open No.2005-290044 contains a nonionic component derived from a polyetherpolyol. The present inventors found that a pigment dispersed using apolymer having such a structure aggregates slowly on a recording medium,resulting in poor color developability.

In Japanese Patent Laid-Open No. 2008-266595, the polyurethane polymeris synthesized only using a polyisocyanate having a ring structure, suchas isophorone diisocyanate or 4,4-dicyclohexylmethane diisocyanate. Itwas found that such a polyurethane polymer has a large number of hardsegments described below because of an interaction between the ringstructures and hydrophobic interaction and becomes excessively rigid,resulting in poor image fastness. Japanese Patent Laid-Open No.2008-266595 also discloses a polyurethane polymer synthesized using apoly(propylene glycol) having a molecular weight of 400. However, it wasfound that the short molecular chain of the polyol results in lowflexibility of the polyurethane polymer relative to its strength,resulting in low scratch resistance and highlighter resistance of animage.

In the polyurethane polymer disclosed in Japanese Patent Laid-Open No.9-291242, the molar ratio of the urethane bond to the urea bond is inthe range of 80/20 to 50/50. The present inventors found that theejection of such an ink containing a polyurethane polymer as disclosedin Japanese Patent Laid-Open No. 9-291242 through an ink jet recordinghead results in the deposition of a polymer component of the ink on asurface of the recording head on which the ejection ports are disposed(hereinafter referred to as a “face”). This phenomenon is referred to as“face wetting”. The polymer component deposited on the face may changethe intended flight direction of an ejected ink droplet. This deflectionmay be small immediately after the ejection of the ink droplet butincrease before reaching a recording medium, causing a problem of thedeflection of some dots in the image (hereinafter referred to as “imagedeflection”). This image deflection occurs not only in a thermal ink jetmethod but also in an ink jet method involving the use of apiezoelectric element.

The polyurethane polymer disclosed in Japanese Patent Laid-Open No.2004-285344 has no urea bond. Thus, the aqueous pigment ink disclosed inJapanese Patent Laid-Open No. 2004-285344 rarely causes imagedeflection. However, as in Japanese Patent Laid-Open No. 2008-266595,the polyurethane polymer is synthesized only using a polyisocyanatehaving a ring structure. This results in poor image fastness.

SUMMARY OF THE INVENTION

Accordingly, aspects of the present invention provide an ink jet inkthat produces an image having high scratch resistance and highlighterresistance, has excellent ink ejection stability, and can reduce imagedeflection caused by face wetting. Aspects of the present invention alsoprovide an ink cartridge containing an ink according to an embodiment ofthe present invention and an ink jet recording method.

These objects described above can be achieved by aspects of the presentinvention described below. An ink jet ink according to one aspect of thepresent invention contains a polyurethane polymer and a pigment. Thepolyurethane polymer has units derived from a polyisocyanate, apolyether polyol having no acid group, and a diol having an acid group.The pigment is dispersed using a polymer other than the polyurethanepolymer. The polyether polyol having no acid group contains at least oneselected from poly(ethylene glycol), poly(propylene glycol),poly(1,2-butylene glycol), and poly(1,3-butylene glycol) and has anumber-average molecular weight of 450 or more and 4,000 or less. Thepolyisocyanate contains a hexamethylene diisocyanate, and the percentage(% by mole) constituted by the unit(s) derived from the hexamethylenediisocyanate with respect to all the units derived from thepolyisocyanate in the polyurethane polymer is 10% by mole or more and90% by mole or less. The ratio of the percentage (% by mole) of urethanebonds in the polyurethane polymer to the percentage (% by mole) of ureabonds in the polyurethane polymer is 85.0/15.0 or more and 100.0/0 orless. The diol having an acid group is at least one selected fromdimethylolpropionic acid and dimethylolbutanoic acid, and the acid valueof the polyurethane polymer resulting from the unit(s) derived from thediol having an acid group is 40 mgKOH/g or more and 140 mgKOH/g or less.

Aspects of the present invention can provide an ink jet ink thatproduces an image having high scratch resistance and highlighterresistance, has excellent ink ejection stability, and can reduce imagedeflection caused by face wetting. Aspects of the present invention canalso provide an ink cartridge containing an ink according to anembodiment of the present invention and an ink jet recording method.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments.

DESCRIPTION OF THE EMBODIMENTS

The present invention will be described in detail with reference to thefollowing embodiments. An ink jet ink (hereinafter referred to simply as“ink”) according to an embodiment of the present invention contains apolyurethane polymer and a pigment. The polyurethane polymer has unitsderived from a polyisocyanate, a polyether polyol having no acid group,and at least one selected from dimethylolpropionic acid anddimethylolbutanoic acid. The pigment is dispersed using a polymer otherthan the polyurethane polymer. The polyether polyol having no acid groupcontains at least one selected from poly(ethylene glycol),poly(propylene glycol), poly(1,2-butylene glycol), and poly(1,3-butyleneglycol) and has a number-average molecular weight of 450 or more and4,000 or less. The polyisocyanate contains a hexamethylene diisocyanate,and the percentage (% by mole) constituted by the unit(s) derived fromthe hexamethylene diisocyanate with respect to all the units derivedfrom the polyisocyanate in the polyurethane polymer is 10% by mole ormore and 90% by mole or less. The ratio of the percentage (% by mole) ofurethane bonds in the polyurethane polymer to the percentage (% by mole)of urea bonds in the polyurethane polymer is 85.0/15.0 or more and100.0/0 or less. The acid value of the polyurethane polymer resultingfrom the unit(s) derived from the at least one selected fromdimethylolpropionic acid and dimethylolbutanoic acid is 40 mgKOH/g ormore and 140 mgKOH/g or less.

The present inventors studied various polymer-dispersed pigment inkscontaining a polyurethane polymer. In accordance with routineprocedures, in order to improve ink ejection stability, polyurethanepolymers having a high acid value or high hydrophilicity were studied.As a result, it was found that some polyurethane polymers providedmarkedly poor image fastness (scratch resistance and highlighterresistance). As a result of extensive studies, the present inventorsfound that the compounds constituting the polyurethane polymer aregreatly responsible for the poor image fastness. The details aredescribed below.

The polyurethane polymer is mainly composed of two segments: a hardsegment and a soft segment. The hard segment is mainly composed of apolyisocyanate, a short-chain diol, and a chain extension agent. Thesoft segment is mainly composed of a polyol. The hard segmentsubstantially contributes to high strength, while the soft segmentsubstantially contributes to flexibility. A micro phase separationstructure of the two segments imparts high strength and flexibility,that is, high elasticity to a film of the polyurethane polymer. Suchfilm characteristics are closely related to image fastness.

As described above, a hydrophilic polyurethane polymer is generallyobtained by increasing the acid value of the polyurethane polymer. Theacid value of the polyurethane polymer depends substantially on thenumber of units derived from a compound having an acid group in thepolyurethane polymer. Thus, in order to increase the acid value of thepolyurethane polymer, the amount of compound having an acid group usedin the synthesis of the polyurethane polymer must be increased. Ingeneral, the compound having an acid group for use in the synthesis ofthe polyurethane polymer may be (a) a polyol having an acid group or (b)a diol having an acid group.

The (a) polyol having an acid group may be an acid-modified polyol, suchas a carboxylic-acid-modified polycaprolactone diol. However, it wasfound that the synthesis of the polyurethane polymer only using a polyolhaving an acid group as a compound having an acid group results inmarked decreases in the scratch resistance and highlighter resistance ofan image. This phenomenon occurred not only in the case of high acidvalues but also in the case of low acid values, that is, in the casethat the amount of polyol having an acid group was small. This isprobably because the polyurethane polymer synthesized using a polyolhaving an acid group has the acid group in the soft segment andconsequently has a poor balance between strength and flexibility. Thus,it was found that even when a polyurethane polymer synthesized onlyusing a polyol having an acid group as a compound having an acid groupis used in an ink, high ink ejection stability and image fastness cannotbe achieved.

The (b) diol having an acid group may be dimethylolpropionic acid(hereinafter referred to as DMPA) or dimethylolbutanoic acid(hereinafter referred to as DMBA). An increase in the amount of DMPA orDMBA to increase the acid value of the polyurethane polymer results inan increase in the number of hydroxy groups. Thus, an increase in theamount of DMPA or DMBA used must be compensated for by a relativedecrease in the amount of polyol component, which has a hydroxy groupreactive with an isocyanate like DMPA or DMBA. This decreases the numberof soft segments, reduces the flexibility of the polyurethane polymer,and increases the rigidity of the resulting polyurethane polymer film,thus lowering image fastness. Thus, it was found that use of apolyurethane polymer having a high acid value due to a diol having anacid group, such as DMPA or DMBA, in an ink can improve ink ejectionstability but lowers image fastness.

The present inventors found that a factor in the markedly poor imagefastness by the use of a polyurethane polymer having a high acid valueor high hydrophilicity is greatly related to the structure of thepolyurethane polymer. The present inventors also found that thepolyurethane polymer must be designed particularly in terms of the twosegment structures, the hard segment and the soft segment, which arecharacteristic of the polyurethane polymer, rather than changing thehydrophilicity and hydrophobicity of the polyurethane polymer withrespect to its acid value.

In consideration of these results, the present inventors examinedvarious polyurethane polymers and found that it is important to use atleast one diol having an acid group selected from DMPA and DMBA as acompound having an acid group used in the synthesis of the polyurethanepolymer. It is also important that the acid value of the polyurethanepolymer resulting from the unit(s) derived from the diol having an acidgroup is 40 mgKOH/g or more and 140 mgKOH/g or less and that, amongpolyether polyols having no acid group, at least one selected frompoly(ethylene glycol), poly(propylene glycol), poly(1,2-butyleneglycol), and poly(1,3-butylene glycol) each having a number-averagemolecular weight of 450 or more and 4,000 or less is used as a polyol.The reasons for these are described below in detail.

In general, a polyester polyol, a polyether polyol, a polycarbonatediol, or a polycaprolactone polyol is used as a polyol in the synthesisof the polyurethane polymer. Among these, a polyether polyol is veryflexible. Thus, even when a diol having an acid group is used in thesynthesis of the polyurethane polymer, a soft segment composed of apolyether polyol rarely causes a reduction in flexibility due to adecrease in the number of soft segments as described above. Thus, evenwith a somewhat high acid value, the polyurethane polymer can have highflexibility. The present inventors found that when the acid value of thepolyurethane polymer resulting from the unit(s) derived from the diolhaving an acid group is 140 mgKOH/g or less the polyurethane polymer canhave high flexibility and excellent image fastness. On the other hand,when the acid value of the polyurethane polymer resulting from theunit(s) derived from the diol having an acid group is less than 40mgKOH/g, the prerequisite ink ejection stability may be lowered. Thus,when a polyether polyol and a diol having an acid group are used, theacid value of the polyurethane polymer resulting from the unit(s)derived from the diol having an acid group must be 40 mgKOH/g or moreand 140 mgKOH/g or less.

It is also found that the flexibility of the polyether polyol alsogreatly depends on its type and molecular weight. The present inventorsstudied the type and the molecular weight of the polyether polyol andfound that the polyether polyol must contain a unit derived from atleast one selected from poly(ethylene glycol), poly(propylene glycol),poly(1,2-butylene glycol), and poly(1,3-butylene glycol) each having apolystyrene-equivalent number-average molecular weight of 450 or moreand 4,000 or less as determined by gel permeation chromatography (GPC).This is because poly(ethylene glycol), poly(propylene glycol),poly(1,2-butylene glycol), and poly(1,3-butylene glycol) areparticularly hydrophilic among polyether polyols. Furthermore, use of apolyol having a molecular weight that results in high strength andflexibility of the polyurethane polymer further improves the scratchresistance and highlighter resistance of an image and ink ejectionstability. When the polyether polyol has a molecular weight of less than450, the polyether polyol has low flexibility because of its shortmolecular chain, and the resulting polyurethane polymer has lowflexibility relative to its strength. On the other hand, when thepolyether polyol has a molecular weight of more than 4,000, thepolyether polyol has excessively high flexibility because of its longmolecular chain, and the resulting polyurethane polymer has low strengthrelative to its flexibility. In both cases, the polyurethane polymer hasa poor balance between strength and flexibility, resulting in lowscratch resistance and highlighter resistance of an image.

As a result of repeated investigations, the present inventors also foundthat, in order to further improve the scratch resistance and highlighterresistance of an image, the polyisocyanate must contain hexamethylenediisocyanate (hereinafter referred to as HDI), and the percentage (% bymole) constituted by the unit(s) derived from HDI with respect to allthe units derived from the polyisocyanate in the polyurethane polymermust be 10% by mole or more and 90% by mole or less. This means that thepolyurethane polymer must have particular percentages (% by mole) of theunit(s) derived from the hexamethylene diisocyanate and the unit(s)derived from a polyisocyanate other than HDI as the unit(s) derived fromthe polyisocyanate. The reason for that is described below.

HDI has a straight chain structure, small steric hindrance, and highmolecular symmetry. In a polyurethane polymer synthesized using HDI,therefore, HDI molecules tend to gather because of hydrogen bonds formedby urethane bonds. Thus, in the polyurethane polymer synthesized usingHDI, hard segments containing the polyisocyanate are localized. Thistends to result in micro phase separation in which the hard segments arepresent in a continuous soft segment to form a sea-island structure,thus markedly increasing the flexibility of the polyurethane polymer. Incontrast, a polyisocyanate that has an intramolecular branched or ringstructure has large steric hindrance and rarely forms a hydrogen bond.However, an interaction between the ring structures and hydrophobicinteraction increase the number of hard segments and markedly increasethe strength of the polyurethane polymer.

Thus, use of HDI and another polyisocyanate in combination impartsflexibility due to HDI and strength due to that other polyisocyanate tothe polyurethane polymer, thereby achieving higher scratch resistanceand highlighter resistance of an image. Thus, the present inventorsstudied the percentage of the unit(s) derived from HDI. As a result, inone embodiment of the present invention, only when the percentage (% bymole) of the unit(s) derived from HDI is 10% by mole or more and 90% bymole or less, the effect of improving strength and the effect ofimproving flexibility due to HDI and the polyisocyanate other than HDIare well balanced. This further improves the scratch resistance andhighlighter resistance of an image.

The polyurethane polymer described above can be used to significantlyimprove ink ejection stability and image fastness (scratch resistanceand highlighter resistance). However, the present inventors found thatthe ejection of such a polyurethane polymer through an ink jet recordinghead may cause another problem of face wetting, which results in imagedeflection.

The present inventors examined various polyurethane polymers and foundthat a polyurethane polymer having a larger number of urea bonds morefrequently causes face wetting. This is probably because two N—H bondsof one urea bond interact with a face to deposit the polyurethanepolymer on the face.

On the basis of these findings, polyurethane polymers having differentmolar ratios of the urethane bond to the urea bond were examined forimage deflection. As a result, it was found that a polyurethane polymerthat has the structure described above and a smaller number of ureabonds less frequently causes image deflection. More specifically, theratio of the percentage (% by mole) of the urethane bond to thepercentage (% by mole) of the urea bond in the polyurethane polymer mustbe 85.0/15.0 or more and 100.0/0 or less. Although a small number ofurea bonds (more specifically, more than 98.5/1.5) can decrease thelikelihood of face wetting, the resulting decrease in the number ofhydrogen bonds formed by the urea bonds may cause deterioration in imagefastness. In accordance with one embodiment of the present invention,however, use of HDI can decrease the likelihood of face wetting whilehigh scratch resistance and highlighter resistance of an image aremaintained. A method for controlling the molar ratio will be describedbelow.

It was also found that use of a polyurethane polymer that has a smallernumber of urea bonds can further improve ink ejection stability. This isprobably because a decrease in the number of hydrogen bonds formed bythe urea bonds results in weaker intramolecular or intermolecularinteraction of the polymer and an increase in the degree of freedom ofthe polymer molecule in the ink, which facilitates the addition of awater molecule to an acid group of the polymer molecule, therebyincreasing hydrophilicity. Although the urethane bond can also form thehydrogen bond, its hydrogen bond strength is smaller than that of theurea bond. Thus, the urethane bond does not prevent the addition ofwater molecule to an acid group of the polymer molecule.

Thus, these constituents can synergistically produce their effects toachieve the advantages according to aspects of the present invention.

Ink Jet Ink

The components of an ink jet ink according to an embodiment of thepresent invention will be described below.

Polyurethane Polymer

A polyurethane polymer for use in an ink according to an embodiment ofthe present invention will be described below in detail.

Polyisocyanate

The term “polyisocyanate”, as used herein, refers to a compound havingtwo or more isocyanate groups. Examples of the polyisocyanate for use inan embodiment of the present invention include, but are not limited to,aliphatic polyisocyanates, alicyclic polyisocyanates, aromaticpolyisocyanates, and araliphatic polyisocyanates. The percentage (% bymass) of the unit(s) derived from the polyisocyanate in the polyurethanepolymer may be 10% by mass or more and 80% by mass or less.

Examples of the aliphatic polyisocyanate include, but are not limitedto, tetramethylene diisocyanate, dodecamethylene diisocyanate,hexamethylene diisocyanate, 2,2,4-trimethylhexamethylene diisocyanate,2,4,4-trimethylhexamethylene diisocyanate, lysine diisocyanate,2-methylpentane-1,5-diisocyanate, and 3-methylpentane-1,5-diisocyanate.Examples of the alicyclic polyisocyanate include, but are not limitedto, isophorone diisocyanate, hydrogenated xylylene diisocyanate,4,4-dicyclohexylmethane diisocyanate, 1,4-cyclohexane diisocyanate,methylcyclohexylene diisocyanate, and1,3-bis(isocyanatomethyl)cyclohexane. Examples of the aromaticpolyisocyanate include, but are not limited to, tolylene diisocyanate,2,2-diphenylmethane diisocyanate, 2,4-diphenylmethane diisocyanate,4,4-diphenylmethane diisocyanate, 4,4-dibenzyl diisocyanate,1,5-naphthylene diisocyanate, xylylene diisocyanate, 1,3-phenylenediisocyanate, and 1,4-phenylene diisocyanate. Examples of thearaliphatic polyisocyanate include, but are not limited to,dialkyldiphenylmethane diisocyanate, tetraalkyldiphenylmethanediisocyanate, and α,α,α,α-tetramethylxylylene diisocyanate. Thesepolyisocyanates may be used alone or in combination. As described above,in one embodiment of the present invention, the polyisocyanate mustcontain HDI, and the percentage (% by mole) constituted by the unit(s)derived from HDI with respect to all the units derived from thepolyisocyanate in the polyurethane polymer must be 10% by mole or moreand 90% by mole or less.

Polyether Polyol Having No Acid Group

As described above, the polyether polyol having no acid group used in anembodiment of the present invention is at least one selected frompoly(ethylene glycol), poly(propylene glycol), poly(1,2-butyleneglycol), and poly(1,3-butylene glycol) each having apolystyrene-equivalent number-average molecular weight of 450 or moreand 4,000 or less as determined by GPC (hereinafter referred to as a“particular polyether polyol”). Among these polyether polyols,poly(propylene glycol) can be used. Use of poly(propylene glycol) canparticularly improve the balance between the strength and theflexibility of the polyurethane polymer film. The percentage (% by mass)of the unit(s) derived from the particular polyether polyol in thepolyurethane polymer may be 0.1% by mass or more and 80.0% by mass orless. The percentage (% by mole) constituted by the unit(s) derived fromthe particular polyether polyol with respect to all the units derivedfrom the polyether polyol in the polyurethane polymer may be 80% by moleor more and 100% by mole or less.

In one embodiment of the present invention, the particular polyetherpolyol may be used in combination with another polyol having no acidgroup other than the particular polyether polyol to synthesize thepolyurethane polymer. In this case, the percentage (% by mole)constituted by the unit(s) derived from the particular polyether polyolwith respect to all the units derived from the polyol having no acidgroup in the polyurethane polymer may be 80% by mole or more and 100% bymole or less. In particular, no castor-oil-modified polyol may be usedin view of ink ejection stability.

Diol Having Acid Group

A polyurethane polymer for use in an ink according to an embodiment ofthe present invention has a unit derived from at least one selected fromDMPA and DMBA as a diol having an acid group. The diol having an acidgroup may be in the form of a salt with an alkali metal, such as Li, Na,or K, or an organic amine, such as ammonia or dimethylamine. These diolsmay be used alone or in combination. The percentage (% by mass) of theunit(s) derived from the diol having an acid group in the polyurethanepolymer may be 5.0% by mass or more and 40.0% by mass or less.

Chain Extension Agent

A chain extension agent is a compound that can react with a residualisocyanate group in a polyisocyanate unit of a urethane prepolymer. Theresidual isocyanate group is an isocyanate group that did not form aurethane bond. In one embodiment of the present invention, a chainextension agent may be used in the synthesis of the polyurethane polymerprovided that the molar ratio of the urethane bond to the urea bond inthe polyurethane polymer is 85.0/15.0 or more and 100.0/0 or less.Examples of the chain extension agent include, but are not limited to,polyvalent amine compounds, such as trimethylolmelamine and derivativesthereof, dimethylolurea and derivatives thereof, dimethylolethylamine,diethanol/methylamine, dipropanolethylamine, dibutanolmethylamine,ethylenediamine, propylenediamine, diethylenetriamine, hexylenediamine,triethylenetetramine, tetraethylenepentamine, isophoronediamine,xylylenediamine, diphenylmethanediamine, hydrogenateddiphenylmethanediamine, and hydrazine, polyamide polyamine, andpolyethylene polyimine. Examples of the chain extension agent alsoinclude, but are not limited to, ethylene glycol, propylene glycol,1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol,1,6-hexanediol, neopentyl glycol, diethylene glycol, triethylene glycol,tetraethylene glycol, dipropylene glycol, tripropylene glycol,poly(ethylene glycol), 3-methyl-1,5-pentanediol,2-butyl-2-ethyl-1,3-propanediol, 1,4-cyclohexanediol,1,4-cyclohexanedimethanol, hydrogenated bisphenol A, glycerin,trimethylolpropane, and pentaerythritol. These chain extension agentsmay be used alone or in combination.

Cross-Linker

A polyurethane polymer for use in an ink according to an embodiment ofthe present invention may be a cross-linked polyurethane polymer. Thecross-linked polyurethane polymer may be produced by using an at leasttrifunctional polyisocyanate, polyol, or chain extension agent, whichfunctions as a cross-linker.

The cross-linked polyurethane polymer is rich in the hard segment of themicro phase separation structure of the polyurethane polymer. This isbecause use of, for example, a trifunctional cross-linker results in theformation of three urethane bonds per cross-linker. This markedlyincreases the strength of the polyurethane polymer. This also increasesthe number of adjacent urethane bonds and accordingly the number ofhydrogen bonds between the urethane bonds. Consequently, the hardsegments gather more closely and easily undergo micro phase separationto form a sea-island structure, maintaining high flexibility of thepolyurethane polymer. Thus, the cross-linked polyurethane polymer hasvery high strength and high flexibility and improves the scratchresistance and highlighter resistance of an image.

Examples of at least trifunctional compound that can be used as across-linker in an embodiment of the present invention include, but arenot limited to, at least trifunctional polyisocyanates, at leasttrifunctional polyols, and at least trifunctional chain extensionagents. The cross-linked polyurethane polymer can be synthesized usingat least one selected from the at least trifunctional compounds.Examples of the at least trifunctional polyisocyanates include, but arenot limited to, polyisocyanurates, adduct-type polyisocyanates, andbiuret polyisocyanates. Examples of the at least trifunctional polyolsinclude, but are not limited to, glycerin, trimethylolpropane,1,2,5-hexanetriol, 1,2,6-hexanetriol, pentaerythritol,polyoxypropylenetriol, and glycol adducts of the polyether polyolsdescribed above. Examples of the at least trifunctional chain extensionagent include, but are not limited to, trimethylolmelamine,diethylenetriamine, triethylenetetramine, and tetraethylenepentamine. Inparticular, at least one selected from glycerin, trimethylolpropane,pentaerythritol, and polyoxypropylenetriol may be used.

Method for Determining Whether Cross-Linked Polyurethane Polymer or Not

The present inventors found that whether the polyurethane polymer is across-linked polyurethane polymer or not can be determined by gelfraction. When a polyurethane polymer is dissolved in a solvent, the gelfraction of the polyurethane polymer is the ratio of the mass ofresidual gel (a cross-linked structure remains as gel) to the mass ofthe polyurethane polymer before dissolved in the solvent. The gelfraction can be measured by the solubility of a polyurethane polymerfilm in a solvent and is a measure of the degree of cross-linking. Apolyurethane polymer having a higher degree of cross-linking has ahigher gel fraction. In one embodiment of the present invention, the gelfraction is measured by the following method to determine whether thepolyurethane polymer is a cross-linked polyurethane polymer or not.

A film of a polyurethane polymer extracted by a method described belowis immersed in tetrahydrofuran (THF) at 23° C. for 24 hours. The mass(A) of a THF-insoluble fraction of the polyurethane polymer film ismeasured and is divided by the mass (B) of the polyurethane polymer filmbefore immersion to calculate a THF gel fraction (A/B×100). In oneembodiment of the present invention, a polyurethane polymer having a THFgel fraction of 88% by mass or more and 100% by mass or less isconsidered to be a cross-linked polyurethane polymer.

Molar Ratio of Urethane Bond to Urea Bond

As described above, the ratio of the percentage (% by mole) of theurethane bond to the percentage (% by mole) of the urea bond in thepolyurethane polymer for use in an ink according to an embodiment of thepresent invention is 85.0/15.0 or more and 100.0/0 or less. In oneembodiment of the present invention, the molar ratio of the urethanebond to the urea bond may be 85.0/15.0 or more and 98.5/1.5 or less.When the molar ratio of the urethane bond to the urea bond is 98.5/1.5or less, the scratch resistance and highlighter resistance of an imageare further improved.

In one embodiment of the present invention, the molar ratio of theurethane bond to the urea bond of the polyurethane polymer may becontrolled by either of the following two methods.

In accordance with a first method, the amount of amine compound used inthe synthesis of the polyurethane polymer is controlled. This is becausethe amine compound reacts with an isocyanate group to form a urea bond.The amine compound may be an amine chain extension agent. Morespecifically, the polyurethane polymer may be synthesized by thefollowing method. First, polyurethane polymers are synthesized usingdifferent amounts of an amine compound. The molar ratio of the urethanebond to the urea bond of each of the polyurethane polymers is measuredby the method described below. On the basis of these results, thecorrelation between the amount of amine compound used and the molarratio of the urethane bond to the urea bond is examined to determine theamounts of raw materials for a polyurethane polymer having apredetermined molar ratio of the urethane bond to the urea bond. Thereason that the correlation between the amount of amine compound usedand the molar ratio is examined in advance is as follows. Even with thesame amine compound, use of different types of other raw materials mayresult in a change in the reaction rate or the like, yieldingpolyurethane polymers having different molar ratios of the urethane bondto the urea bond.

In accordance with a second method, the percentage of the unreactedisocyanate group is controlled in phase inversion of the polyurethanepolymer to water. Water reacts with an isocyanate group to form a ureabond. More specifically, the polyurethane polymer is synthesized by thefollowing method. First, the percentage of the unreacted isocyanategroup in the synthesis of the polyurethane polymer is measured with aFourier transform infrared spectrophotometer (FT-IR). When thepercentage of the unreacted isocyanate group reaches the predeterminedmolar ratio of the urethane bond to the urea bond, ion-exchanged wateris added to the reaction system. For example, in the synthesis of apolyurethane polymer having a molar ratio of the urethane bond to theurea bond of 95.0/5.0, ion-exchanged water is added when the percentageof the unreacted isocyanate group is 5% by mole. The percentage of theunreacted isocyanate group may be controlled by the reaction time or theinitial amount of polyisocyanate. In the examples described below, themolar ratio of the urethane bond to the urea bond of the polyurethanepolymer was controlled by the second method.

Characteristics of Polyurethane Polymer

The percentage (% by mass) of the polyurethane polymer for use in an inkaccording to an embodiment of the present invention may be 0.1% by massor more and 10.0% by mass or less of the total mass of the ink. Lessthan 0.1% by mass of the polyurethane polymer may have an insufficienteffect of improving the scratch resistance and highlighter resistance ofan image. More than 10.0% by mass of the polyurethane polymer may havean insufficient effect of improving ink ejection stability. The ink mayfurther contain another polymer without compromising the advantagesaccording to aspects of the present invention.

The ratio of the polyurethane polymer content (% by mass) of the ink tothe pigment content (% by mass) based on the total mass of the inkdescribed below may be 0.05 or more and 2.00 or less. A mass ratio ofless than 0.05 may result in an insufficient effect of improving thescratch resistance and highlighter resistance of an image. A mass ratioof more than 2.00 may result in an insufficient effect of improving inkejection stability.

A polyurethane polymer for use in an ink according to an embodiment ofthe present invention may have a polystyrene-equivalent weight-averagemolecular weight (Mw) of more than 30,000 and 150,000 or less asdetermined by GPC. When the polystyrene-equivalent weight-averagemolecular weight (Mw) is 30,000 or less, the polyurethane polymer mayhave low strength and an insufficient effect of improving the scratchresistance and highlighter resistance of an image. When thepolystyrene-equivalent weight-average molecular weight (Mw) is more than150,000, the ink tends to have a high viscosity, and this may result inan insufficient effect of improving ink ejection stability.

A polyurethane polymer for use in an ink according to an embodiment ofthe present invention may have an acid value of 40 mgKOH/g or more and150 mgKOH/g or less. As described above, the acid value of thepolyurethane polymer resulting from the unit(s) derived from the diolhaving an acid group must be 40 mgKOH/g or more and 140 mgKOH/g or less.The acid value of the polyurethane polymer resulting from the unit(s)derived from the diol having an acid group may be 45 mgKOH/g or more and100 mgKOH/g or less.

Method for Synthesizing Polyurethane Polymer

A method for synthesizing a polyurethane polymer according to anembodiment of the present invention may be any known method, includingthe following method. A polyisocyanate, a polyol having no acid group,and a diol having an acid group are allowed to react to synthesize aurethane prepolymer having a terminal isocyanate group. An acid group ofthe urethane prepolymer is then neutralized with a neutralizing agent.The neutralized urethane prepolymer is further allowed to react in anaqueous solution containing a chain extension agent or a cross-linker.An organic solvent remaining in the system, if any, may be removed.

The present inventors found that, when two polyisocyanates (a firstpolyisocyanate and a second polyisocyanate) are used to synthesize apolyurethane polymer having a decreased number of urea bonds, ahigh-molecular-weight polyurethane polymer can be produced by thefollowing method while the molar ratio of the urethane bond to the ureabond is controlled. More specifically, the first polyisocyanate, apolyol having no acid group, and a diol having an acid group are allowedto react. The reaction is continued until the percentage of theunreacted isocyanate group in the reaction system becomes zero asmeasured with an FT-IR, yielding a prepolymer solution. This prepolymercontains no isocyanate group of the first polyisocyanate. The secondpolyisocyanate is added to the prepolymer solution and is allowed toreact until the percentage of the unreacted isocyanate group in thereaction system reaches a predetermined value as measured with an FT-IR.A chain extension agent or a cross-linker is then added to theprepolymer solution and is allowed to react to yield a polyurethanepolymer solution. The phase inversion of the polyurethane polymersolution to water yields a polyurethane polymer dispersion having apredetermined molar ratio of the urethane bond to the urea bond. In thissynthesis method, when the first polyisocyanate is isophoronediisocyanate, the scratch resistance and highlighter resistance of animage are improved. Whether a polyurethane polymer is synthesized bythis synthesis method or not may be determined by the following method.When the molar ratio X of the urethane bond formed between the firstpolyisocyanate and the diol having an acid group to the firstpolyisocyanate is larger than the molar ratio Y of the urethane bondformed between the second polyisocyanate and the diol having an acidgroup to the second polyisocyanate, the polyurethane polymer isconsidered to be synthesized by the synthesis method described above. Xand Y can be determined by nuclear magnetic resonance (NMR) as describedbelow.

Method for Analyzing Polyurethane Polymer

The composition, the molecular weight, and the acid value of theresulting polyurethane polymer can be analyzed by examining the sedimentand the supernatant liquid after the centrifugation of the ink. Sincethe pigment is insoluble in organic solvents, the polyurethane polymercan also be isolated by solvent extraction. Although the ink itself canbe analyzed, the isolation of the polyurethane polymer can improve theaccuracy of measurement. More specifically, after the centrifugation ofthe ink at 80,000 rpm, the supernatant liquid is precipitated withhydrochloric acid and is dried.

(1) Polyurethane Polymer Composition

The dried precipitate is dissolved in deuterated dimethyl sulfoxide(deuterated DMSO) and is subjected to proton nuclear magnetic resonance(¹H-NMR). The types of the polyisocyanate, the polyether polyol havingno acid group, and the diol having an acid group can be determined bythe peak positions of the proton nuclear magnetic resonance and themeasurement of the dried precipitate by pyrolysis-gas chromatography.The component ratio can be calculated from the ratio of the integralvalues of chemical shift peaks. The percentage constituted by theunit(s) derived from HDI with respect to all the units derived from thepolyisocyanate in the polyurethane polymer can be calculated by thismethod.

(2) Method for Measuring Molar Ratio of Urethane Bond to Urea Bond ofPolyurethane Polymer

The molar ratio of the urethane bond to the urea bond of thepolyurethane polymer can be determined by the ratio of the integralvalue of the peaks of the urethane bond to the integral value of thepeaks of the urea bond of the polyurethane polymer precipitate dissolvedin deuterated DMSO measured by carbon nuclear magnetic resonance(¹³C-NMR). The peak positions of the urethane bond and the urea bonddepend on the types of raw material compounds of the polyurethanepolymer (a polyisocyanate, a polyether polyol having no acid group, anda diol having an acid group). Thus, the peak positions of the urethanebond and the urea bond for the raw material compounds of thepolyurethane polymer must be examined by the following method.

First, the raw material compounds of the polyurethane polymer (apolyisocyanate, a polyether polyol having no acid group, and a diolhaving an acid group) are prepared. (i) A reaction product between thepolyisocyanate and the polyether polyol having no acid group, (ii) areaction product between the polyisocyanate and the diol having an acidgroup, and (iii) a reaction product between the polyisocyanate and waterare obtained. Each of the products is then dried, is dissolved indeuterated DMSO, and is subjected to ¹³C-NMR measurement. The peakposition of the urethane bond in each of the reaction products can beidentified from the results for (i) and (ii). The peak position of theurea bond in each of the reaction products can be identified from theresult for (iii). For example, when the polyisocyanate is isophoronediisocyanate, the peak position of the urethane bond is approximately155 ppm, and the peak position of the urea bond is approximately 158ppm.

(3) Method for Measuring Acid Value of Polyurethane Polymer

The acid value of the polyurethane polymer can be measured bytitrimetry. In the examples described below, the acid value of thepolyurethane polymer was measured by potentiometric titration of thepolymer dissolved in THF with an automatic potentiometric titrator AT510(manufactured by Kyoto Electronics Manufacturing Co., Ltd.) using apotassium hydroxide-ethanol titrant.

The acid value of the diol having an acid group (DMPA or DMBA) can bemeasured by the following method. First, a polyurethane polymerprecipitate dissolved in deuterated DMSO is subjected to ¹³C-NMRmeasurement. The molar ratio of DMPA or DMBA to another compound havingan acid group (for example, acrylic acid) is calculated by the ratio ofthe peak of the quaternary carbon atom of DMPA or DMBA to the integralvalue of the peak of an acid group of that other compound. The acidvalue of the polyurethane polymer is multiplied by the molar ratio ofDMPA or DMBA to calculate the acid value of the diol having an acidgroup, DMPA or DMBA. The peak position of an acid group derived from thequaternary carbon atom of DMPA or DMBA is approximately 65 or 60 ppm,respectively. When no compound having an acid group other than DMPA orDMBA is used, the acid value of the diol having an acid group can bemeasured by the titrimetry.

(4) Method for Measuring Average Molecular Weight of PolyurethanePolymer

The average molecular weight of the polyurethane polymer can be measuredby GPC. In GPC measurement, an apparatus Alliance GPC 2695 (manufacturedby Waters), four columns of Shodex KF-806M (manufactured by Showa DenkoK.K.) in series, and a R1 (refractive index) detector were used. Theaverage molecular weight was calculated using polystyrene standardsamples PS-1 and PS-2 (manufactured by Polymer Laboratories).

Polymer-Dispersed Pigment

An ink according to an embodiment of the present invention contains apigment dispersed using a polymer other than the polyurethane polymer.

Pigment

Examples of the pigment for use in an ink according to an embodiment ofthe present invention include, but are not limited to, inorganic andorganic pigments, such as carbon black. Any known pigment can be used inan ink jet ink according to an embodiment of the present invention. Thepresent inventors found that the polyurethane polymer is physicallyadsorbed on inorganic pigments more easily than organic pigments. Use ofinorganic pigments therefore more effectively improves the scratchresistance and highlighter resistance of an image. The amount (% bymass) of pigment in the ink ranges from 0.1% to 15.0% by mass, such asfrom 1.0% to 8.0% by mass, based on the total mass of the ink. A pigmentcontent of less than 1.0% by mass may result in insufficient opticaldensity. A pigment content of more than 8.0% by mass may result in poorink jet characteristics, such as sticking resistance.

Method for Determining Whether Pigment is Dispersed with Polymer or Not

A method for determining whether the pigment is dispersed using apolymer or not is described below. An ink concentrated or diluted to asolid content of approximately 10% by mass is centrifuged at 12,000 rpmfor one hour. After a water-soluble organic solvent and a polymer thatdoes not contribute to the dispersion of the pigment are transferred toa liquid layer, precipitated components, including the pigment, arecollected. If the precipitated components, including the pigment,include a polymer, the pigment can be considered to be apolymer-dispersed pigment. A polymer included as a main component in theprecipitated components, including the pigment, is a polymer that isinvolved in the dispersion of the pigment (polymer dispersant). Apolymer contained as a main component in the liquid layer is a polymerthat does not contribute to the dispersion of the pigment. In oneembodiment of the present invention, a polymer included as a maincomponent in the precipitated components, including the pigment, may bea polymer other than the polyurethane polymer. This means that theprecipitated components, including the pigment, may include thepolyurethane polymer, but the amount of the polymer other than thepolyurethane polymer may be larger than the polyurethane polymercontent.

Polymer Dispersant

A polymer dispersant for use in an ink according to an embodiment of thepresent invention may be any known polymer dispersant that has been usedin ink jet inks. In one embodiment of the present invention, the polymerdispersant may be water-soluble. The sentence “a polymer iswater-soluble”, as used herein, means that the polymer neutralized withan equivalent amount of alkaline has no particle size. The polymerdispersant may be composed of at least two monomers described below. Atleast one of the monomers may be a hydrophilic monomer. Examples of themonomers include, but are not limited to, styrene, vinylnaphthalene,aliphatic alcohol esters of α,β-ethylenically unsaturated carboxylicacids, acrylic acid, methacrylic acid, maleic acid, itaconic acid,fumaric acid, vinyl acetate, vinylpyrrolidone, acrylamide, andderivatives thereof. The hydrophilic monomer may be acrylic acid ormethacrylic acid. In particular, in one embodiment of the presentinvention, the polymer dispersant may be a copolymer containing unitsderived from styrene and (meth)acrylic acid. The polymer dispersant maybe a block copolymer, a random copolymer, a graft copolymer, or a saltthereof. The polymer dispersant may also be a natural polymer, such asrosin, shellac, or starch. In one embodiment of the present invention,the polyurethane polymer is not the polymer dispersant.

The polymer dispersant may have a polystyrene-equivalent weight-averagemolecular weight of 1,000 or more and 30,000 or less, such as 3,000 ormore and 15,000 or less, as determined by GPC. In one embodiment of thepresent invention, the polymer dispersant may have an acid value of 90mgKOH/g or more and 180 mgKOH/g or less. The polymer dispersant havingan acid value in this range can improve the dispersion stability of thepigment and ink ejection stability. The polymer dispersant content (% bymass) may be 0.1% by mass or more and 5.0% by mass or less, such as 0.5%by mass or more and 3.0% by mass or less, of the total mass of the ink.The ratio of the pigment content (% by mass) to the polymer content (%by mass) in the ink may be 0.3 or more and 5.0 or less, such as 0.5 ormore and 2.0 or less. In embodiments of the present invention, thecomponent contents in the mass ratio are based on the total mass of theink.

Aqueous Medium

An ink according to an embodiment of the present invention may containwater or an aqueous medium, such as a mixed solvent of water and awater-soluble organic solvent. The amount (% by mass) of water-solubleorganic solvent in the ink may be 3.0% by mass or more and 50.0% by massor less of the total mass of the ink. The water-soluble organic solventmay be any known solvent generally used in ink jet inks. Examples of thewater-soluble organic solvent include, but are not limited to, alkylalcohols having 1 to 4 carbon atoms, amides, ketones, keto-alcohols,ethers, poly(alkylene glycol)s, glycols, alkylene glycols in which thealkylene group has 2 to 6 carbon atoms, polyhydric alcohols, alkyl etheracetates, alkyl ethers of polyhydric alcohols, nitrogen-containingcompounds, and sulfur-containing compounds. These water-soluble organicsolvents may be used alone or in combination. Water can be deionizedwater (ion-exchanged water). The water content (% by mass) of the inkmay be 50.0% by mass or more and 95.0% by mass or less of the total massof the ink. The ink viscosity at 25° C. may be 6 cps or less. The inkviscosity can be controlled through the composition and the amount ofthe aqueous medium. An ink viscosity of more than 6 cps at 25° C. mayresult in an insufficient effect of improving ink ejection stability.

Other Additive Agents

In addition to the components described above, an ink according to anembodiment of the present invention may further contain a water-solubleorganic compound that is solid at normal temperature, for example, apolyhydric alcohol, such as trimethylolpropane or trimethylolethane,urea, or a urea derivative, such as ethylene urea. An ink according toan embodiment of the present invention may also contain an additiveagent, such as a polymer other than the polyurethane polymer, asurfactant, a pH-adjusting agent, an anticorrosive, a preservative, afungicide, an antioxidant, a reduction inhibitor, an evaporationaccelerator, and/or a chelator. In the case that the ink contains apolymer other than the polyurethane polymer and the polymer dispersant,the total polymer content of the ink may be 0.01% by mass or more and10.00% by mass or less of the total mass of the ink.

Ink Cartridge

An ink cartridge according to an embodiment of the present inventionincludes an ink storage portion for storing an ink according to anembodiment of the present invention. The ink storage portion may includean ink chamber and a chamber for housing a negative-pressure-generatingmember. The ink chamber can store liquid ink. Thenegative-pressure-generating member can store ink by the action of anegative pressure. Alternatively, an ink cartridge according to anembodiment of the present invention may include no ink chamber andinclude an ink storage portion that includes anegative-pressure-generating member for storing the whole ink.Alternatively, an ink cartridge according to an embodiment of thepresent invention may include an ink storage portion and a recordinghead.

Ink Jet Recording Method

An ink jet recording method according to an embodiment of the presentinvention involves ejecting an ink according to an embodiment of thepresent invention from an ejection port of a recording head onto arecording medium by an ink jet method in response to recording signals.The ink may be ejected from an ejection port of a recording head by theaction of thermal energy. In one embodiment of the present invention,the face of the recording head may be subjected to water-repellentfinishing. The face of the recording head may be subjected towater-repellent finishing by any method. For example, the face of therecording head may be treated with a water repellent made of a siliconematerial or a fluorinated material. Examples of the water repellentinclude, but are not limited to, KP-801 (manufactured by Shin-EtsuChemical Co., Ltd.), Defensa (manufactured by DIC Corp.), Cytop CTX-105,805 (manufactured by Asahi Glass Co., Ltd.), and Teflon (registeredtrademark) AF (manufactured by Du Pont). A fluorine-containing silanecompound may also be used as a water repellent. The term “recording”, asused herein, includes recording on a recording medium with an inkaccording to an embodiment of the present invention or printing on asubstrate having little permeability, such as a glass substrate, aplastic substrate, or a nonpermeable film, with an ink according to anembodiment of the present invention. The recording medium may be plainpaper or glossy paper. Glossy paper includes a porous ink-absorbinglayer on a permeable support (such as paper). The porous ink-absorbinglayer contains an inorganic pigment and a binder. In one embodiment ofthe present invention, the recording medium may be glossy paper.

An ink jet recording method according to an embodiment of the presentinvention may include a process (A) of ejecting an ink through an inkjet recording head onto a recording medium and a process (B) of applyinga liquid composition described below to the recording medium such thatthe liquid composition at least partly overlaps the ink. The process (A)may be followed or preceded by the process (B). The same processes maybe performed twice or more; for example, the process (A), the process(B), and then the process (A), or the process (B), the process (A), andthen the process (B) may be performed. In particular, performing theprocess (A) after the process (B) has a large effect of improving thescratch resistance and optical density of an image.

Liquid Composition

An ink according to an embodiment of the present invention may be usedin combination with a liquid composition that can destabilize thedispersion of pigment in the ink and reduce the solubility of thepolyurethane polymer. The pigment reacts with a reactant in the liquidcomposition to aggregate rapidly and remain on the surface of arecording medium, thus increasing optical density. The polyurethanepolymer also reacts with a reactant in the liquid composition. Thisreduces the solubility of the polyurethane polymer and causesprecipitation of the polyurethane polymer. Thus, the polyurethanepolymer is present in the vicinity of the aggregated pigment andimproves the scratch resistance and highlighter resistance of an image.The phrase “destabilize the dispersion of pigment in the ink”, as usedherein, means that pigment particles aggregate as a result of areduction in electrostatic repulsion by which the pigment particles aredispersed in the ink. The phrase “reduce the solubility of thepolyurethane polymer”, as used herein, means that the polyurethanepolymer, which has been dissolved (dispersed) in the ink by the additionof a water molecule to an acid group of the polyurethane polymer(hydration), is insolubilized by the prevention of hydration.

In one embodiment of the present invention, the liquid composition maybe colorless, milk white, or white so as not to affect an image recordedby the ink. Thus, the ratio Amax/Amin of the maximum absorbance Amax tothe minimum absorbance Amin in a visible light wavelength in the rangeof 400 to 780 nm may be 1.0 or more and 2.0 or less. This means that theabsorption peak is substantially absent or very small in the visiblelight wavelength region. A liquid composition according to an embodimentof the present invention may contain no coloring material. Theabsorbance may be measured after the liquid composition is diluted. Thisis because the maximum absorbance Amax and the minimum absorbance Aminof the liquid composition are proportional to the dilution ratio, andtherefore the Amax/Amin is independent of the dilution ratio. Thecomponents of the liquid composition are described below.

Reactant

In one embodiment of the present invention, the liquid composition maycontain a reactant that can destabilize the dispersion of pigment andreduce the solubility of the polyurethane polymer in the ink. Morespecifically, the reactant may be a polyvalent metal ion or an organicacid. The reactant content (% by mass) of the liquid composition may be3.0% by mass or more and 20.0% by mass or less of the total mass of theliquid composition. Less than 3.0% by mass of the reactant cannotsufficiently destabilize the pigment dispersion and may have aninsufficient effect of improving optical density. More than 20.0% bymass of the reactant may cause the precipitation of the reactant andresult in poor ink jet characteristics.

Polyvalent Metal Ion

In one embodiment of the present invention, the polyvalent metal ion ofthe liquid composition may be at least divalent. Examples of thedivalent metal ion include, but are not limited to, alkaline-earthmetals, such as beryllium, magnesium, calcium, strontium, barium, andradium. Examples of at least trivalent metal ion include, but are notlimited to, aluminum, yttrium, zirconium, iron, and other transitionmetal ions. In one embodiment of the present invention, the polyvalentmetal ion may be added in the form of salt, such as hydroxide, chloride,or nitrate, to the liquid composition. The polyvalent metal ion may alsobe a dissociated ion.

In one embodiment of the present invention, at least one selected fromcalcium ion, aluminum ion, and yttrium ion may be used in terms ofreactivity. In particular, calcium ion can be used. A nitrate may beused in terms of the solubility of the salt. One example of nitrate iscalcium nitrate.

Organic Acid

The term “organic acid”, as used herein, refers to an acid of an organiccompound. In one embodiment of the present invention, the organic acidof the liquid composition may be a monocarboxylic acid, such as formicacid, acetic acid, propionic acid, or butyric acid; a dicarboxylic acid,such as oxalic acid, malonic acid, succinic acid, or glutaric acid; or ahydroxycarboxylic acid, such as malic acid or tartaric acid. In oneembodiment of the present invention, the organic acid may be added inthe form of alkali metal ion salt to the liquid composition or may alsobe a dissociated ion.

When the reactant is an organic acid, the liquid composition may have apH of 3.5 or more and 5.5 or less. When the liquid composition has a pHof less than 3.5, this may result in the acid corrosion of a member ofan ink jet recording apparatus. When the liquid composition has a pH ofmore than 5.5, this may result in an insufficient effect of improvingthe scratch resistance and highlighter resistance of an image. The pH ofthe liquid composition is measured at 25° C. and can be measured with acommon pH meter. When the reactant is an organic acid, the liquidcomposition may have pH-buffering action. The sentence “the liquidcomposition has pH-buffering action”, as used herein, means that anequivalent mixture of the liquid composition and the ink hassubstantially the same pH as the liquid composition. The phrase“substantially the same pH as the liquid composition”, as used herein,refers to a pH change of less than 0.1.

Aqueous Medium and Another Additive Agent

The liquid composition may contain water or an aqueous medium, such as amixed solvent of water and a water-soluble organic solvent. The amount(% by mass) of water-soluble organic solvent in the liquid compositionmay be 3.0% by mass or more and 50.0% by mass or less of the total massof the liquid composition. The water-soluble organic solvent may be thewater-soluble organic solvent that can be used in the ink describedabove. Water can be deionized water (ion-exchanged water). The watercontent (% by mass) of the liquid composition may be 50.0% by mass ormore and 95.0% by mass or less of the total mass of the liquidcomposition. The liquid composition may contain the additive agent(s)described above for the ink. In particular, in one embodiment of thepresent invention, the pH of the liquid composition may be adjusted to3.5 or more and 5.5 or less with a pH-adjusting agent. Examples of thepH-adjusting agent include, but are not limited to, organic acids, suchas acetic acid and methanesulfonic acid, inorganic acids, such assulfuric acid and nitric acid, and bases, such as alkali metalhydroxide.

EXAMPLES

Aspects of the present invention will be further described in thefollowing examples and comparative examples. However, the presentinvention is not limited to these examples. Unless otherwise specified,“part” in the examples is based on mass. The following areabbreviations.

IPDI: isophorone diisocyanate

MDI: dicyclohexylmethane diisocyanate

HDI: hexamethylene diisocyanate

PPG: poly(propylene glycol)

PEG: poly(ethylene glycol)

P(12BG): poly(1,2-butylene glycol)

P(13BG): poly(1,3-butylene glycol)

PTMG: poly(tetramethylene glycol)

PC: polycarbonate diol

PES: polyester polyol

DMPA: dimethylolpropionic acid

DMBA: dimethylolbutanoic acid

TMP: trimethylolpropane

Gly: glycerin

PE: pentaerythritol

PPT: polyoxypropylenetriol (Actcol 32-160, manufactured by TakedaPharmaceutical Co., Ltd.)

Preparation of Polyurethane Polymer Dispersion Preparation ofPolyurethane Polymer Dispersions PU-1 to PU-35

A polyisocyanate (A part and B part), a polyol (C part), a diol havingan acid group (D part), and methyl ethyl ketone (300 parts) in afour-neck flask equipped with a thermometer, an agitator, a nitrogeninlet, and a reflux condenser were allowed to react in a nitrogen gasatmosphere at 80° C. for six hours. A cross-linker (E part) was added tothe flask and was allowed to react at 80° C. until a predetermined molarratio of the urethane bond to the urea bond was obtained while thepercentage of the unreacted isocyanate group was measured with an FT-IR.The molar ratio of the urethane bond to the urea bond was controlled bythe method described above in detail. After completing the reaction andcooling to 40° C., ion-exchanged water was added to the flask, andaqueous potassium hydroxide was added to the flask while stirring with ahomomixer at a high speed. The polymer solution was heated under reducedpressure to evaporate methyl ethyl ketone, yielding polyurethane polymerdispersions PU-1 to PU-35 each having a solid content of 20% by mass anda weight-average molecular weight of 35,000 or more. Table 1 shows thepreparation conditions of the polyurethane polymer dispersions. The acidvalue, the molar ratio of the urethane bond to the urea bond, and thegel fraction of each of the polyurethane polymers were measured by themethod described above. Table 2 shows the properties of the polyurethanepolymer dispersions.

Preparation of Polyurethane Polymer Dispersion PU-36

IPDI (49.5 parts), PPG (number-average molecular weight: 2,000) (103.7parts), DMPA (28.7 parts), and methyl ethyl ketone (300 parts) in afour-neck flask equipped with a thermometer, an agitator, a nitrogeninlet, and a reflux condenser were allowed to react in a nitrogen gasatmosphere at 80° C. for seven hours. The percentage of the unreactedisocyanate group was zero as measured with an FT-IR. HDI (14.3 parts)was added to the flask and was allowed to react at 80° C. A cross-linkerTMP (3.8 parts) was added to the flask and was allowed to react at 80°C. until the molar ratio of the urethane bond to the urea bond reached95.0/5.0 while the percentage of the unreacted isocyanate group wasmeasured with an FT-IR. After completing the reaction and cooling to 40°C., ion-exchanged water was added to the flask, and aqueous potassiumhydroxide was added to the flask while stirring with a homomixer at ahigh speed. The polymer solution was heated under reduced pressure toevaporate methyl ethyl ketone, yielding a polyurethane polymerdispersion PU-36 having a solid content of 20% by mass and aweight-average molecular weight of 50,000. The acid value, the molarratio of the urethane bond to the urea bond, and the gel fraction ofeach of the polyurethane polymers were measured by the method describedabove. Table 2 shows the characteristics of the polyurethane polymerdispersion PU-36.

TABLE 1 Preparation conditions for polyurethane (PU) polymer dispersionsPU Diol having acid polymer Polyisocyanate Polyol group Cross-linkerdispersion A B Number-average C D E No. Type (parts) Type (parts) Typemolecular weight (parts) Type (parts) Type (parts) PU-1 IPDI 32.0 HDI24.2 PPG 2000 111.2 DMPA 28.7 TMP 3.8 PU-2 MDI 37.5 HDI 24.0 PPG 2000106.0 DMPA 28.7 TMP 3.8 PU-3 IPDI 31.9 HDI 24.1 PPG 2000 108.5 DMBA 31.7TMP 3.8 PU-4 IPDI 32.0 HDI 24.2 PPG 2000 111.2 DMPA 28.7 TMP 3.8 PU-5IPDI 32.0 HDI 24.2 PPG 2000 111.2 DMPA 28.7 TMP 3.8 PU-6 IPDI 25.3 HDI19.2 PPG 2000 132.6 DMPA 19.1 TMP 3.8 PU-7 IPDI 27.0 HDI 20.4 PPG 2000127.3 DMPA 21.5 TMP 3.8 PU-8 IPDI 45.5 HDI 34.4 PPG 2000 68.5 DMPA 47.8TMP 3.8 PU-9 IPDI 59.0 HDI 44.6 PPG 2000 25.7 DMPA 66.9 TMP 3.8 PU-10IPDI 46.9 HDI 35.5 PPG 450 85.2 DMPA 28.7 TMP 3.8 PU-11 IPDI 29.1 HDI22.0 PPG 4000 116.4 DMPA 28.7 TMP 3.8 PU-12 IPDI 57.1 HDI 4.8 PPG 2000105.7 DMPA 28.7 TMP 3.8 PU-13 IPDI 6.5 HDI 44.1 PPG 2000 117.0 DMPA 28.7TMP 3.8 PU-14 IPDI 31.6 HDI 23.9 PPG 2000 112.8 DMPA 28.7 TMP 3.0 PU-15IPDI 32.9 HDI 24.9 PPG 2000 108.0 DMPA 28.7 TMP 5.7 PU-16 IPDI 32.0 HDI24.2 PEG 2000 111.2 DMPA 28.7 TMP 3.8 PU-17 IPDI 32.0 HDI 24.2 P(12BG)2000 111.2 DMPA 28.7 TMP 3.8 PU-18 IPDI 32.0 HDI 24.2 P(13BG) 2000 111.2DMPA 28.7 TMP 3.8 PU-19 IPDI 32.9 HDI 24.9 PPG 2000 110.0 DMPA 28.7 Gly3.8 PU-20 IPDI 31.5 HDI 23.9 PPG 2000 111.2 DMPA 28.7 PE 3.8 PU-21 IPDI31.5 HDI 23.9 PPG 2000 111.2 DMPA 28.7 PPT 3.8 PU-22 IPDI 32.0 HDI 24.2PPG 2000 111.2 DMPA 28.7 TMP/Gly 2.2/1.6 PU-23 IPDI 30.3 HDI 22.9 PPG2000 118.1 DMPA 28.7 — — PU-24 IPDI 32.0 HDI 24.2 PPG 2000 111.2 DMPA28.7 TMP 3.8 PU-25 IPDI 23.6 HDI 17.9 PPG 2000 137.9 DMPA 16.7 TMP 3.8PU-26 IPDI 60.6 HDI 45.9 PPG 2000 20.4 DMPA 69.3 TMP 3.8 PU-27 IPDI 48.6HDI 36.8 PPG 400 82.1 DMPA 28.7 TMP 3.8 PU-28 IPDI 28.1 HDI 21.2 PPG6000 118.2 DMPA 28.7 TMP 3.8 PU-29 IPDI 32.0 HDI 24.2 PES 2000 111.2DMPA 28.7 TMP 3.8 PU-30 IPDI 32.0 HDI 24.2 PC 2000 111.2 DMPA 28.7 TMP3.8 PU-31 IPDI 32.0 HDI 24.2 PTMG 2000 111.2 DMPA 28.7 TMP 3.8 PU-32IPDI 57.7 HDI 4.3 PPG 2000 105.5 DMPA 28.7 TMP 3.8 PU-33 IPDI 5.8 HDI44.6 PPG 2000 117.2 DMPA 28.7 TMP 3.8 PU-34 IPDI 68.1 — 0 PPG 2000 99.4DMPA 28.7 TMP 3.8 PU-35 IPDI 68.1 — 0 PPG 2000 99.4 DMPA 28.7 TMP 3.8

TABLE 2 Characteristics of polyurethane (PU) polymer dispersions Acidvalue (mgKOH/g) Percentage constituted by PU Acid value of PU unit(s)derived from HDI with polymer Acid value polymer resulting from Urethanerespect to all units derived dispersion of PU unit(s) derived frombond/urea bond from polyisocyanate in PU Gel fraction No. polymer diolhaving acid group (molar ratio) polymer (mol %) (%) PU-1  60 60 95.0/5.050 94 PU-2  60 60 95.0/5.0 50 94 PU-3  60 60 95.0/5.0 50 94 PU-4  60 60 85.0/15.0 50 94 PU-5  60 60 100.0/0   50 94 PU-6  40 40 95.0/5.0 50 94PU-7  45 45 95.0/5.0 50 94 PU-8  100 100 95.0/5.0 50 94 PU-9  140 14095.0/5.0 50 94 PU-10 60 60 95.0/5.0 50 94 PU-11 60 60 95.0/5.0 50 94PU-12 60 60 95.0/5.0 10 94 PU-13 60 60 95.0/5.0 90 94 PU-14 60 6095.0/5.0 50 88 PU-15 60 60 95.0/5.0 50 100 PU-16 60 60 95.0/5.0 50 94PU-17 60 60 95.0/5.0 50 94 PU-18 60 60 95.0/5.0 50 94 PU-19 60 6095.0/5.0 50 94 PU-20 60 60 95.0/5.0 50 94 PU-21 60 60 95.0/5.0 50 94PU-22 60 60 95.0/5.0 50 94 PU-23 60 60 95.0/5.0 50 81 PU-24 60 60 84.0/16.0 50 94 PU-25 35 35 95.0/5.0 50 94 PU-26 145 145 95.0/5.0 50 94PU-27 60 60 95.0/5.0 50 94 PU-28 60 60 95.0/5.0 50 94 PU-29 60 6095.0/5.0 50 94 PU-30 60 60 95.0/5.0 50 94 PU-31 60 60 95.0/5.0 50 94PU-32 60 60 100.0/0   9 94 PU-33 60 60 100.0/0   91 94 PU-34 60 6095.0/5.0 0 94 PU-35 60 60 95.0/5.0 0 94 PU-36 60 60 95.0/5.0 50 94

Preparation of Pigment Dispersion Preparation of Pigment Dispersion A

A styrene-acrylic acid copolymer having an acid value of 200 mgKOH/g anda weight-average molecular weight of 10,000 was neutralized with 10% bymass aqueous potassium hydroxide. 10 parts of carbon black having aspecific surface area of 210 m²/g and a DBP absorption of 74 mL/100 g,20 parts of the neutralized styrene-acrylic acid copolymer (solidcontent), and 70 parts of water were mixed. This mixture was dispersedwith a sand grinder for one hour, was centrifuged to remove coarseparticles, and was filtrated under pressure through a microfilter havinga pore size of 3.0 μm (manufactured by FUJIFILM Co.). Through theseprocedures, a pigment dispersion A in which carbon black was dispersedin water by the polymer was prepared. The pigment dispersion A had apigment content (solid content) of 10.0% by mass and a pH of 10.0. Thepigment had an average particle size of 120 nm.

Preparation of Pigment Dispersion B

A commercially available Cab-O-Jet 400 (manufactured by Cabot Corp.) waswell stirred in water to yield a pigment dispersion B. Cab-O-Jet 400 isa self-dispersing carbon black pigment having a hydrophilic group on thesurface thereof. The pigment dispersion B had a pigment content (solidcontent) of 15.0% by mass and a pH of 9.0. The pigment had an averageparticle size of 130 nm.

Preparation of Pigment Dispersion C

A pigment dispersion C in which carbon black was dispersed in water by apolyurethane polymer was prepared in the same manner as in the pigmentdispersion A except that the styrene-acrylic acid copolymer was replacedby the polyurethane polymer dispersion PU-1. The pigment dispersion Chad a pigment content (solid content) of 10.0% by mass and a pH of 10.0.The pigment had an average particle size of 120 nm.

Preparation of Ink

A combination of the pigment dispersion and the polyurethane polymerdispersion shown in Table 3 was mixed with the following components. Theamount of ion-exchanged water (the remainder) was such that the totalamount of the components of the ink was 100.0% by mass.

Pigment dispersion see Table 3 Polyurethane polymer dispersion (polymercontent see Table 3 (solid content) was 20.0% by mass) Glycerin 9.0% bymass Diethylene glycol 5.0% by mass Triethylene glycol 5.0% by massAcetylenol (trade name) E100 (surfactant, 0.1% by mass manufactured byKawaken Fine Chemicals Co., Ltd.) Ion-exchanged water the remainder

The mixture was well dispersed and was passed through a microfilter(manufactured by Fujifilm Corp.) having a pore size of 3.0 μm underpressure to prepare an ink.

TABLE 3 Preparation conditions for inks Pigment dispersion PU polymerdispersion PU polymer Content Content content/pigment Example No. No(mass %) No. (mass %) content (times) Example 1 A 30.00 PU-1  15.00 1.00Example 2 A 30.00 PU-1  0.75 0.05 Example 3 A 30.00 PU-1  30.00 2.00Example 4 A 30.00 PU-1  0.60 0.04 Example 5 A 30.00 PU-1  31.50 2.10Example 6 A 30.00 PU-2  15.00 1.00 Example 7 A 30.00 PU-3  15.00 1.00Example 8 A 30.00 PU-4  15.00 1.00 Example 9 A 30.00 PU-5  15.00 1.00Example 10 A 30.00 PU-6  15.00 1.00 Example 11 A 30.00 PU-7  15.00 1.00Example 12 A 30.00 PU-8  15.00 1.00 Example 13 A 30.00 PU-9  15.00 1.00Example 14 A 30.00 PU-10 15.00 1.00 Example 15 A 30.00 PU-11 15.00 1.00Example 16 A 30.00 PU-12 15.00 1.00 Example 17 A 30.00 PU-13 15.00 1.00Example 18 A 30.00 PU-14 15.00 1.00 Example 19 A 30.00 PU-15 15.00 1.00Example 20 A 30.00 PU-16 15.00 1.00 Example 21 A 30.00 PU-17 15.00 1.00Example 22 A 30.00 PU-18 15.00 1.00 Example 23 A 30.00 PU-19 15.00 1.00Example 24 A 30.00 PU-20 15.00 1.00 Example 25 A 30.00 PU-21 15.00 1.00Example 26 A 30.00 PU-22 15.00 1.00 Example 27 A 30.00 PU-23 15.00 1.00Example 28 A 30.00 PU-36 15.00 1.00 Comparative example 1 A 30.00 PU-2415.00 1.00 Comparative example 2 A 30.00 PU-25 15.00 1.00 Comparativeexample 3 A 30.00 PU-26 15.00 1.00 Comparative example 4 A 30.00 PU-2715.00 1.00 Comparative example 5 A 30.00 PU-28 15.00 1.00 Comparativeexample 6 A 30.00 PU-29 15.00 1.00 Comparative example 7 A 30.00 PU-3015.00 1.00 Comparative example 8 A 30.00 PU-31 15.00 1.00 Comparativeexample 9 A 30.00 PU-32 15.00 1.00 Comparative example 10 A 30.00 PU-3315.00 1.00 Comparative example 11 C 30.00 PU-1  15.00 1.00 Referenceexample 1 A 30.00 PU-34 15.00 1.00 Reference example 2 B 20.00 PU-357.50 0.50

Polyurethane polymer dispersions and inks according to ComparativeExamples 12 to 16 were prepared as described below.

Comparative Example 12

An ink according to Comparative Example 12 was prepared as describedbelow with reference to Example 1 of Japanese Patent Laid-Open No.2004-285344. 35 parts of poly(tetramethylene ether) glycol, 3 parts oftriethylene glycol, 16 parts of dimethylolpropionic acid, 9 parts oftetramethylenexylylene diisocyanate, and 25 parts of isophoronediisocyanate were allowed to react in acetone in a nitrogen stream toyield a urethane prepolymer. The urethane prepolymer was added dropwiseand dispersed in deionized water containing 12 parts of triethylamine.The acetone was removed under vacuum to yield a polyurethane polymerdispersion PU-36. PU-36 had a solid content of 30% by mass, aweight-average molecular weight of 20,840, and an acid value of 69mgKOH/g. 10.4 parts of a 25% by mass aqueous solution of a potassiumsalt of a styrene-acrylic acid copolymer Joncryl 683 (manufactured byJohnson Polymer; acid value 160 mgKOH/g, weight-average molecular weight8,000), 71.5 parts of ion-exchanged water, 5 parts of glycerin, 0.1parts of Proxel GXL(S) (manufactured by Avecia Ltd.), and 13 parts ofcarbon black MCF 88 (manufactured by Mitsubishi Chemical Corp.) weredispersed in a bead mill with 0.8-mm zirconia beads such that theaverage diameter of dispersed particles became 61.9 nm. Aftercentrifugation, coarse particles were removed by a 5-μm filter to yielda carbon black dispersion. The carbon black dispersion was mixed with 26parts of PU-36. 32.5 parts of triethylene glycol monobutyl ether, 32.5parts of glycerin, 3.25 parts of Surfynol 465 (manufactured by AirProducts and Chemicals, Inc.), and 130.75 parts of ion-exchanged waterwere added to the mixture to yield an ink according to ComparativeExample 12.

Comparative Example 13

An ink according to Comparative Example 13 was prepared as describedbelow with reference to Example 1 of Japanese Patent Laid-Open No.2008-179657. A four-neck flask equipped with a thermometer, an agitator,a nitrogen inlet, and a condenser tube was charged with 95 g of Placcel205BA (manufactured by Daicel Chemical Industries, Ltd.), 11 g oftrimethylolpropane, 120 g of methyl ethyl ketone, and 0.54 g of1,4-diazabicyclo[2.2.2]octane. Placcel 205BA is acarboxylic-acid-modified polycaprolactone diol, in whichdimethylolbutanoic acid is modified with a lactone. After agitation for30 minutes, 74 g of isophorone diisocyanate was added to the four-neckflask. After agitation at room temperature in a nitrogen atmosphere forone hour, a reaction was performed at 70° C. for four hours. After thereaction, the product was cooled to room temperature to yield a 60% bymass urethane prepolymer solution. 17.1 g of 50% by mass aqueouspotassium hydroxide and 350 g of ion-exchanged water were added to 250 gof the urethane prepolymer solution in the four-neck flask and werestirred at room temperature for 30 minutes. The mixture was heated to80° C. in a nitrogen atmosphere and was subjected to a chain extensionreaction for two hours. After the reaction, methyl ethyl ketone and partof water were removed with a rotatory evaporator and an aspirator.Ion-exchanged water was then added such that the amount of recovery was429 g to yield a 35% by mass polyurethane polymer dispersion PU-37.PU-37 had an acid value of 70 mgKOH/g and a weight-average molecularweight of 47,000.

After 3 kg of carbon black MA-100 (manufactured by Mitsubishi ChemicalCorp.) was mixed with 10 kg of water, the mixture was added to 4.5 kg ofa sodium hypochlorite solution (available chlorine concentration 12%).After agitation at a temperature in the range of 100 to 105° C. for 10hours, the resulting product was filtered. Dried wet crystals werewashed with water and were dried at 80° C. to yield 2.5 kg of oxidizedcarbon black. The oxidized carbon black was mixed with ion-exchangedwater to prepare a slurry. The slurry was dialyzed with a poly(methylmethacrylate) dialysis module Filtryzer B3-20A (manufactured by TorayIndustries, Inc.) to remove sodium ions and chloride ions in theoxidized carbon black and was dried to yield dialyzed oxidized carbonblack. 120 g of the dialyzed oxidized carbon black, 180 g of triethyleneglycol monobutyl ether, and 700 g of ion-exchanged water were mixed in aHomo Disper agitator to prepare a slurry. A beaker containing the slurrywas connected to a circulation-type bead mill DYNO-Mill KDL-A(manufactured by Willy A. Bachofen AG) through a tube. The slurry wasdispersed with zirconia beads having a diameter of 0.3 mm at 1,600 rpmfor three hours to prepare an aqueous black pigment dispersion. 0.7 g of50% by mass aqueous potassium hydroxide, 0.5 g of ion-exchanged water,and 7.1 g of the polyurethane polymer dispersion PU-37 (solid content35% by mass) were then added to 41.7 g of the aqueous black pigmentdispersion to prepare an aqueous black pigment dispersion (pigmentcontent: 10% by mass, polyurethane polymer content: 5% by mass). 25.8 gof ion-exchanged water, 3.5 g of glycerin, 1.7 g of 2-pyrrolidone, and1.5 g of ethylene glycol were then added to 17.5 g of the aqueous blackpigment dispersion to prepare an ink according to Comparative Example13.

Comparative Example 14

An ink according to Comparative Example 14 was prepared as describedbelow with reference to Comparative Example 3 of Japanese PatentLaid-Open No. 9-291242. 184.9 parts of poly(tetramethylene glycol)(molecular weight 3,000, hydroxyl value 38 mgKOH/g), 15.1 parts ofpoly(ethylene glycol) (molecular weight 2,000, hydroxyl value 56mgKOH/g), 21.5 parts of 2,2-dimethylolpropionic acid, and 200.0 parts ofmethyl ethyl ketone in a reactor equipped with a thermometer, anagitator, a reflux condenser tube, and a nitrogen gas-inlet pipe wereheated to 50° C. in a nitrogen atmosphere while stirring. After theaddition of 71.6 parts of isophorone diisocyanate, the mixture wasallowed to react at 80° C. for 2.5 hours to yield anisocyanate-group-containing prepolymer solution. After cooling to 30°C., a solution of 6.9 parts of propylene glycol and 229 parts of methylethyl ketone was added dropwise and was allowed to react at 70° C. Aliquid mixture of 9.8 parts of 28% by mass aqueous ammonia and 900 partsof water was then added dropwise. The removal of the solvent yielded apolyurethane polymer dispersion PU-38 having no urea bond. PU-38 had anacid value of 30 mgKOH/g, a pH of 8.5, and a solid content of 25.0% bymass.

250.0 parts of an ethylene-acrylic acid copolymer Primacor 5983(manufactured by Dow Chemical Japan Ltd., acid value 156 mgKOH/g, meltindex 500 g/10 minutes) were neutralized with 278.6 parts of 10% by massaqueous sodium hydroxide and 471.4 parts of ion-exchanged water and werethen dissolved by heating to prepare an aqueous ethylene-acrylic acidcopolymer solution having a solid content of 25% by mass and a pH of9.0. 40.3 parts of the aqueous ethylene-acrylic acid copolymer solutionand 17.3 parts of the polyurethane polymer dispersion PU-38 were mixedto prepare a polymer composition. 15.0 parts of a phthalocyanine bluepigment Lionol Blue FG-7350 (manufactured by Toyo Ink Co., Ltd.), 0.3parts of a silicone antifoaming agent, 2.0 parts of a polyethylene waxdispersion (solid content: 40% by mass), 4.5 parts of ethanol, and 20.6parts of ion-exchanged water were then added to 57.6 parts of thepolymer composition to prepare an ink according to Comparative Example14.

Comparative Example 15

An ink according to Comparative Example 15 was prepared as describedbelow with reference to Example 1 of Japanese Patent Laid-Open No.2008-280363. 202 parts of a castor-oil-modified diol (manufactured byHokoku Corp., number-average molecular weight 732), 32 parts of acastor-oil-modified diol (manufactured by Hokoku Corp., number-averagemolecular weight 431), 79 parts of dimethylolpropionic acid, 43 parts ofpolyoxyethylene ether glycol PEG#600 (manufactured by NOF Corp.), 13parts of propylene glycol, 76 parts of 1,6-hexamethylene diisocyanate,155 parts of hydrogenated MDI (dicyclohexylmethane diisocyanate), and400 parts of methyl ethyl ketone in a reactor equipped with an agitator,a reflux condenser tube, a nitrogen inlet, and a thermometer were heatedto 75° C. After one hour, 0.3 parts of dibutyltin laurate was added tothe mixture. The mixture was kept at 75° C. so that the isocyanate groupwas decreased to 0.1% by mole or less. The reaction product was thencooled to 45° C. or less. 40 parts of 25% by mass aqueous ammonia and1,400 parts of pure water were added while stirring for phase inversion.Methyl ethyl ketone was removed under vacuum while stirring at atemperature in the range of 20 to 60° C. to yield a polyurethane polymerdispersion PU-39 having a solid content of 33% by mass and a pH of 6.7.

500 parts of a self-dispersing carbon black dispersion Aqua-Black 174(manufactured by Tokai Carbon Co., Ltd., solid content 20% by mass) wasadded to 3 parts of the polyurethane polymer dispersion PU-39 and wasstirred at room temperature for two hours to prepare an ink according toComparative Example 15.

Comparative Example 16

An ink according to Comparative Example 16 was prepared as describedbelow with reference to Example 2 of Japanese Patent Laid-Open No.2008-266595.290 g of methyl ethyl ketone, 67 g of dimethylolpropionicacid, and 222 g of isophorone diisocyanate in a four-neck flask equippedwith a thermometer, an agitator, a nitrogen inlet, and a condenser tubewere allowed to react in a nitrogen gas atmosphere at 70° C. for twohours to yield a reaction product. 289 g of the reaction product, 170 gof poly(oxytetramethylene) glycol (molecular weight 650, hydroxyl value173), 0.001 g of dibutyltin dilaurate, and 170 g of methyl ethyl ketonein a four-neck flask equipped with a thermometer, an agitator, anitrogen inlet, and a condenser tube were allowed to react in a nitrogengas atmosphere at 80° C. for 16 hours. 5 g of methanol was added to theflask to terminate the reaction. The resulting polyurethane polymer hadan acid value of 46 and a weight-average molecular weight of 50,000. Theresulting polymer solution was neutralized with the same molar amount ofsodium hydroxide as the initial dimethylolpropionic acid. Water was thenadded to the polymer solution for phase inversion and emulsification.After methyl ethyl ketone was removed under vacuum, water was added toyield a polyurethane polymer dispersion PU-40 having a solid content of20% by mass.

Six parts of a 50% methyl ethyl ketone solution of a benzylmethacrylate-methacrylic acid copolymer (acid value 130, mass-averagemolecular mass 12,000), 7.8 parts of 5% aqueous potassium hydroxide, 10parts of phthalocyanine pigment Fastogen Blue TGR (manufactured by DICCorp.), and 23.8 parts of water were mixed in a 250-mL plastic bottle.These amounts were determined such that the copper phthalocyaninepigment was 10 parts, the non-volatile content of the polymer was 30% bymass of the pigment, the 5% aqueous potassium hydroxide could completelyneutralize the polymer, and water could dilute the liquid mixture to anon-volatile content of 30%. The liquid mixture was dispersed with apaint conditioner (with zirconia beads having a diameter of 0.5 mm) fortwo hours. After dispersion and removal of the zirconia beads, methylethyl ketone was removed with an evaporator. After centrifugation for 30minutes to remove coarse particles, pure water was added to yield anaqueous pigment dispersion having a non-volatile content of 20%. Amixture of 19.9 parts of the aqueous pigment dispersion, 7.5 parts ofthe polyurethane polymer dispersion PU-40, 10.0 parts of diethyleneglycol monobutyl ether, 15.0 parts of diethylene glycol, 0.8 parts ofSurfynol 465 (manufactured by Air Products and Chemicals, Inc.), and46.8 parts of water was passed through a filter having a pore size of 6μm to prepare an ink according to Comparative Example 16.

Evaluation

In the following evaluation, the criteria AAA to B refer to acceptablelevels, and the criteria C and D refer to unacceptable levels. Theevaluation was performed with an ink jet recording apparatus PIXUSiP3100 (manufactured by CANON KABUSHIKI KAISHA). The recordingconditions included a temperature of 23° C., a relative humidity of 55%,and an ink droplet weight of 28 ng (within ±10%). With the ink jetrecording apparatus, an image that was recorded under the conditionsunder which approximately 28 ng of one ink droplet was applied to a unitarea of 1/600 inches 1/600 inches at a resolution of 600 dpi 600 dpi wasassumed to have a print duty of 100%.

Scratch Resistance of Image

An ink cartridge filled with the ink prepared as described above wasplaced in the ink jet recording apparatus. A 1.0 inch*0.5 inches solidimage (print duty 100%) was recorded on a PPC sheet GF-500 (manufacturedby CANON KABUSHIKI KAISHA). Ten minutes and one day after the recording,a Silbon paper and a weight having a contact pressure of 40 g/cm² wereplaced on the solid image, and the solid image and the Silbon paper wererubbed together. After the Silbon paper and the weight were removed,smudges on the solid image and a transfer to the white ground of theSilbon paper were visually inspected. The following are evaluationcriteria for the scratch resistance of an image. Table 4 shows theresults.

AAA: No smudge on the white ground was observed in the test after 10minutes and the test after one day.

AA: Little smudge on the white ground was observed in the test after 10minutes. No smudge on the white ground was observed in the test afterone day.

A: Little smudge on the white ground was observed in the test after 10minutes and the test after one day.

B: Unnoticeable smudges on the white ground were observed in the testafter 10 minutes. Little smudge on the white ground was observed in thetest after one day.

C: Smudges on the white ground were observed in the test after 10minutes and the test after one day.

D: Noticeable smudges on the white ground were observed in the testafter 10 minutes and the test after one day.

Highlighter Resistance of Image

An ink cartridge filled with the ink prepared as described above wasplaced in the ink jet recording apparatus. A vertical rule having awidth of 1/10 inches was recorded on a PPC sheet GF-500 (manufactured byCANON KABUSHIKI KAISHA). Five minutes and one day after the recording,the vertical rule was traced with a yellow highlighter OPTEX2(manufactured by ZEBRA Co., Ltd.), and immediately after that a line wasdrawn with the yellow highlighter on a white ground of a recordingmedium to check for contamination of the pen nib and contamination ofthe line on the white ground. The following are evaluation criteria forthe highlighter resistance of an image. Table 4 shows the results.

AAA: No contamination of the pen nib and no contamination of the line onthe white ground were observed in the test after five minutes and thetest after one day.

AA: In the test after five minutes, although coloring on the pen nib wasobserved, little contamination of the line on the white ground wasobserved. In the test after one day, there were no contamination of thepen nib and no contamination of the line on the white ground.

A: In the test after five minutes and the test after one day, althoughcoloring on the pen nib was observed, little contamination of the lineon the white ground was observed.

B: In the test after five minutes, although coloring on the pen nib wasobserved, the contamination of the line on the white ground wasunnoticeable. In the test after one day, although coloring on the pennib was observed, little contamination of the line on the white groundwas observed.

C: In the test after five minutes and the test after one day, there werecoloring of the pen nib and contamination of the line on the whiteground.

D: In the test after five minutes and the test after one day, there weresignificant coloring of the pen nib and significant contamination of theline on the white ground.

Ink Ejection Stability

An ink cartridge filled with the ink prepared as described above wasplaced in the ink jet recording apparatus. A 19 cm*26 cm solid image(print duty 100%) was recorded on ten GF-500 PPC sheets (manufactured byCANON KABUSHIKI KAISHA). The solid image on the tenth sheet was visuallyinspected to evaluate ink ejection stability. The following areevaluation criteria for ink ejection stability. Table 4 shows theresults.

A: Neither white streak nor faint streak was observed, and the solidimage was properly recorded.

B: A few unnoticeable white streaks or faint streaks were observed.

C: Ink ejection was unstable, and white streaks or faint streaks wereobserved on the image.

Prevention of Image deflection

An ink cartridge filled with the ink prepared as described above wasplaced in the ink jet recording apparatus. A 19 cm*26 cm solid image(print duty 100%) was recorded on two GF-500 PPC sheets (manufactured byCANON KABUSHIKI KAISHA). After the ink jet recording apparatus was leftto stand for 30 minutes, recording the image on two GF-500 PPC sheetswas performed ten times. A nozzle check pattern was then recorded withthe ink jet recording apparatus (PIXUS iP3100). The nozzle check patternwas visually inspected for image deflection. After the removal of arecording head, the face of the recording head was observed with amicroscope to check for face wetting. The following are evaluationcriteria for image deflection and face wetting. Table 4 shows theresults.

AA: No disorder in the nozzle check pattern was observed, indicating theprevention of image deflection. No face wetting was observed.

A: No disorder in the nozzle check pattern was observed, indicating theprevention of image deflection. Slight face wetting was observed.

B: An unnoticeable disorder in the nozzle check pattern was observed,indicating the prevention of image deflection. A little face wetting wasobserved.

C: A significant disorder in the nozzle check pattern was observed, andno satisfactory image was obtained, indicating the occurrence of imagedeflection. Face wetting was also observed.

TABLE 4 Evaluation results Evaluation of ink Evaluation of imagePrevention Scratch Highlighter Ejection of image Example No. resistanceresistance stability deflection Example 1 AAA AAA A A Example 2 AAA AAAA A Example 3 AAA AAA A A Example 4 AA AA A A Example 5 AAA AAA B AExample 6 AAA AAA A A Example 7 AAA AAA A A Example 8 AAA AAA A BExample 9 AAA AAA A A Example 10 AAA AAA B A Example 11 AAA AAA A AExample 12 AAA AAA A A Example 13 AA AA A A Example 14 AAA AAA A AExample 15 AAA AAA A A Example 16 AA AA A A Example 17 AA AA A A Example18 AAA AAA A A Example 19 AAA AAA A A Example 20 AA AA A A Example 21AAA AAA B A Example 22 AAA AAA B A Example 23 AAA AAA A A Example 24 AAAAAA A A Example 25 AAA AAA A A Example 26 AAA AAA A A Example 27 AA AA AA Example 28 AAA AAA A A Comparative example 1 AAA AAA A C Comparativeexample 2 AAA AAA C A Comparative example 3 D D A A Comparative example4 D D A A Comparative example 5 D D A A Comparative example 6 A A C AComparative example 7 A A C A Comparative example 8 AAA AAA C AComparative example 9 C C A A Comparative example 10 C C A A Comparativeexample 11 B C C A Comparative example 12 D D B A Comparative example 13D D C A Comparative example 14 D D C A Comparative example 15 C C C AComparative example 16 D D B A Reference example 1 A A A AA Referenceexample 2 B B A A

Three minutes after the recording, the results of the evaluation ScratchResistance of Image and Highlighter Resistance of Image for Example 28were better than the results for Example 1.

Preparation of Ink Ink 1

The pigment dispersion A and the polyurethane polymer dispersion PU-1were mixed with other components as described below.

Pigment dispersion A (pigment content (solid content) 30.0% by mass was10.0% by mass) Polyurethane polymer dispersion PU-1 (polymer content15.0% by mass (solid content) was 20.0% by mass) Glycerin  9.0% by massDiethylene glycol  5.0% by mass Triethylene glycol  5.0% by massAcetylenol (trade name) E100 (surfactant,  1.0% by mass manufactured byKawaken Fine Chemicals Co., Ltd.) Ion-exchanged water 35.0% by mass

The mixture was well dispersed and was passed through a microfilter(manufactured by Fujifilm Corp.) having a pore size of 3.0 μm underpressure to prepare an ink 1.

Preparation of Liquid Composition Liquid Composition 1

The following components were mixed. The amount of ion-exchanged water(the remainder) was such that the total amount of the components of theink was 100.0% by mass.

Reactant: calcium nitrate 5.0% by mass Glycerin 5.0% by mass1,5-pentanediol 5.0% by mass Trimethylene glycol 7.0% by mass Acetylenol(trade name) E100 (surfactant, 0.1% by mass manufactured by Kawaken FineChemicals Co., Ltd.) Ion-exchanged water the remainder

The mixture was well dispersed and was passed through a microfilter(manufactured by Fujifilm Corp.) having a pore size of 3.0 μm underpressure to prepare a liquid composition 1.

Liquid Composition 2

A liquid composition 2 was prepared in the same manner as in the liquidcomposition 1 except that the reactant calcium nitrate was replaced byaluminum nitrate.

Liquid Composition 3

A liquid composition 3 was prepared in the same manner as in the liquidcomposition 1 except that the reactant calcium nitrate was replaced byyttrium nitrate.

Liquid Composition 4

A liquid composition 4 was prepared in the same manner as in the liquidcomposition 1 except that the reactant calcium nitrate was replaced bymagnesium nitrate.

Liquid Composition 5

The following components were mixed. The amount of ion-exchanged water(the remainder) was such that the total amount of the components of theink was 100.0% by mass.

Reactant: sodium citrate 10.0% by mass Glycerin  5.0% by mass1,5-pentanediol  5.0% by mass Trimethylolpropane  7.0% by mass NIKKOLBC-20 (surfactant, manufactured by Nikko  1.0% by mass Chemicals Co.,Ltd.) Ion-exchanged water the remainder

The mixture was well dispersed and was passed through a microfilter(manufactured by Fujifilm Corp.) having a pore size of 3.0 μm underpressure. The mixture was then treated with sulfuric acid to prepare aliquid composition 5 having a pH of 4.0.

Liquid Composition 6

A liquid composition 6 was prepared in the same manner as in the liquidcomposition 5 except that the pH was 3.0.

Liquid Composition 7

A liquid composition 7 was prepared in the same manner as in the liquidcomposition 5 except that the pH was 3.5.

Liquid Composition 8

A liquid composition 8 was prepared in the same manner as in the liquidcomposition 5 except that the pH was 5.5.

Liquid Composition 9

A liquid composition 9 was prepared in the same manner as in the liquidcomposition 5 except that the pH was 6.0.

The absorbances of the liquid compositions 1 to 9 without dilution weremeasured with a Hitachi double-beam spectrophotometer U-2900(manufactured by Hitachi High-Technologies Corp.). With these liquidcompositions, the ratio Amax/Amin of the maximum absorbance Amax to theminimum absorbance Amin in a wavelength in the range of 400 to 780 nmwas 1.0 or more and 2.0 or less.

Evaluation

In the following evaluation, the criteria AAA to B refer to acceptablelevels, and the criteria C and D refer to unacceptable levels. TheOptical Density, Scratch Resistance of Image, and Highlighter Resistanceof Image were evaluated with an ink jet recording apparatus PIXUSPro9500 (manufactured by CANON KABUSHIKI KAISHA). The recordingconditions included a temperature of 23° C. and a relative humidity of55%. An ink cartridge separately filled with the ink and the liquidcomposition listed in Table 5 was placed in the ink jet recordingapparatus PIXUS Pro9500. The ink was contained in a chamber for yellow,and the liquid composition was contained in a chamber for gray.Recording was performed unidirectionally from the home position to theopposite position. The width of the recording corresponded to the nozzlewidth of the head. After the liquid composition was applied, the ink wasapplied on the liquid composition in the same pass to form an image.With the ink jet recording apparatus, an image recorded under theconditions under which approximately 16 ng of one ink droplet wasapplied to a unit area (one pixel) of 1/600 inches 1/600 inches at aresolution of 600 dpi* 600 dpi was assumed to have an ink print duty of100%. An image recorded under the conditions under which approximately 7ng of one droplet of the liquid composition was applied to one pixel wasassumed to have a liquid composition print duty of 100%.

Optical Density

An ink cartridge filled with the ink and the liquid composition preparedas described above was placed in the ink jet recording apparatus. A 2cm*2 cm solid image (ink and liquid composition print duties 100%) wasprinted on four types of recording media: a PPC sheet GF-500(manufactured by CANON KABUSHIKI KAISHA), a PPC sheet 4024 (manufacturedby Xerox Corp.), a PPC sheet Bright White (manufactured byHewlett-Packard Co.), and a PPC sheet Hammermill Jet Print (manufacturedby International Paper). After the solid image was left to stand for oneday, the optical density was measured with a reflection densitometerMacbeth RD-918 (manufactured by Macbeth). The following are evaluationcriteria for optical density. Table 5 shows the results.

AA: The average optical density of the four recording media was 1.40 ormore, and the highest optical density was 1.60 or more.

A: The average optical density of the four recording media was 1.40 ormore, but the highest optical density was 1.55 or more and less than1.60.

B: The average optical density of the four recording media was 1.40 ormore, but the highest optical density was less than 1.55.

C: The average optical density of the four recording media was less than1.40.

Scratch Resistance of Image

An ink cartridge filled with the ink and the liquid composition preparedas described above was placed in the ink jet recording apparatus. A 1.0inch*0.5 inches solid image (ink and liquid composition print duties100%) was recorded on a PPC sheet GF-500 (manufactured by CANONKABUSHIKI KAISHA).

(1) Evaluation of Scratch Resistance of Image Three Minutes afterRecording

Three minutes after the recording, a Silbon paper and a weight having acontact pressure of 40 g/cm² were placed on the solid image, and thesolid image and the Silbon paper were rubbed together. After the Silbonpaper and the weight were removed, smudges on the solid image and atransfer to the white ground of the Silbon paper were visuallyinspected. The following are evaluation criteria for the scratchresistance of an image. Table 5 shows the results.

AA: No smudge on the white ground was observed in the test after threeminutes.

A: Little smudge on the white ground was observed in the test afterthree minutes.

B: Unnoticeable smudges on the white ground were observed in the testafter three minutes.

C: Smudges on the white ground were observed in the test after threeminutes.

D: Noticeable smudges on the white ground were observed in the testafter three minutes.

(2) Evaluation of Scratch Resistance of Image 10 Minutes and One Dayafter Recording

Ten minutes and one day after the recording, a Silbon paper and a weighthaving a contact pressure of 40 g/cm² were placed on the solid image,and the solid image and the Silbon paper were rubbed together. After theSilbon paper and the weight were removed, smudges on the solid image anda transfer to the white ground of the Silbon paper were visuallyinspected. The following are evaluation criteria for the scratchresistance of an image. Table 5 shows the results.

AAA: No smudge on the white ground was observed in the test after 10minutes and the test after one day.

AA: Little smudge on the white ground was observed in the test after 10minutes. No smudge on the white ground was observed in the test afterone day.

A: Little smudge on the white ground was observed in the test after 10minutes and the test after one day.

B: Unnoticeable smudges on the white ground were observed in the testafter 10 minutes. Little smudge on the white ground was observed in thetest after one day.

C: Smudges on the white ground were observed in the test after 10minutes and the test after one day.

D: Noticeable smudges on the white ground were observed in the testafter 10 minutes and the test after one day. Highlighter Resistance ofImage

An ink cartridge filled with the ink and the liquid composition preparedas described above was placed in the ink jet recording apparatus. Avertical rule having a width of 1/10 inches was recorded on a PPC sheetGF-500 (manufactured by CANON KABUSHIKI KAISHA).

(1) Evaluation of Highlighter Resistance of Image Three Minutes afterRecording

Three minutes after the recording, the vertical rule was traced with ayellow highlighter OPTEX2 (manufactured by ZEBRA Co., Ltd.), andimmediately after that a line was drawn with the yellow highlighter on awhite ground of a recording medium to check for contamination of the pennib and contamination of the line on the white ground. The following areevaluation criteria for the highlighter resistance of an image. Table 5shows the results.

AA: No contamination of the pen nib and no contamination of the line onthe white ground were observed in the test after three minutes.

A: In the test after three minutes, although coloring on the pen nib wasobserved, little contamination of the line on the white ground wasobserved.

B: In the test after three minutes, although coloring on the pen nib wasobserved, the contamination of the line on the white ground wasunnoticeable.

C: In the test after three minutes, there were coloring of the pen niband contamination of the line on the white ground.

D: In the test after three minutes, there were significant coloring ofthe pen nib and significant contamination of the line on the whiteground.

(2) Evaluation of Highlighter Resistance of Image Five Minutes and OneDay after Recording

Five minutes and one day after the recording, the vertical rule wastraced with a yellow highlighter OPTEX2 (manufactured by ZEBRA Co.,Ltd.), and immediately after that a line was drawn with the yellowhighlighter on a white ground of a recording medium to check forcontamination of the pen nib and contamination of the line on the whiteground. The following are evaluation criteria for the highlighterresistance of an image. Table 5 shows the results.

AAA: No contamination of the pen nib and no contamination of the line onthe white ground were observed in the test after five minutes and thetest after one day.

AA: In the test after five minutes, although coloring on the pen nib wasobserved, little contamination of the line on the white ground wasobserved. In the test after one day, there were no contamination of thepen nib and no contamination of the line on the white ground.

A: In the test after five minutes and the test after one day, althoughcoloring on the pen nib was observed, little contamination of the lineon the white ground was observed.

B: In the test after five minutes, although coloring on the pen nib wasobserved, the contamination of the line on the white ground wasunnoticeable. In the test after one day, although coloring on the pennib was observed, little contamination of the line on the white groundwas observed.

C: In the test after five minutes and the test after one day, there werecoloring of the pen nib and contamination of the line on the whiteground.

D: In the test after five minutes and the test after one day, there weresignificant coloring of the pen nib and significant contamination of theline on the white ground.

TABLE 5 Combination of liquid composition and ink and evaluation resultsEvaluation of image Scratch resistance Highlighter resistance (1) 3 min(2) 10 min and (1) 3 min (2) 5 min and Liquid composition Ink Opticalafter 1 day after after 1 day after Example No. No. density recordingrecording recording recording Example 29 Liquid composition 1 Ink 1 AAAAA AA AAA AA Example 30 Liquid composition 2 Ink 1 AA AAA AA AAA AAExample 31 Liquid composition 3 Ink 1 AA AAA AA AAA AA Example 32 Liquidcomposition 4 Ink 1 AA AAA A AAA A Example 33 Liquid composition 5 Ink 1AA AAA AA AAA AA Example 34 Liquid composition 6 Ink 1 AA AAA AA AAA AAExample 35 Liquid composition 7 Ink 1 AA AAA AA AAA AA Example 39 Liquidcomposition 8 Ink 1 AA AAA AA AAA AA Example 37 Liquid composition 9 Ink1 AA AAA B AAA B

When the liquid composition had a pH of less than 3.5, as in Example 34,the wettability of a member in contact with the liquid composition wasinferior to the liquid composition having a pH of 3.5 or more (forexample, Example 35).

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2011-075883 filed Mar. 30, 2011 and No. 2011-075885 filed Mar. 30, 2011,which are hereby incorporated by reference herein in their entirety.

1. An ink jet ink, comprising: a polyurethane polymer and a pigment, thepolyurethane polymer having units derived from a polyisocyanate, apolyether polyol having no acid group, and a diol having an acid group,the pigment being dispersed using a polymer other than the polyurethanepolymer, wherein the polyether polyol having no acid group contains atleast one selected from poly(ethylene glycol), poly(propylene glycol),poly(1,2-butylene glycol), and poly(1,3-butylene glycol) and has anumber-average molecular weight of 450 or more and 4,000 or less, thepolyisocyanate contains a hexamethylene diisocyanate, and the percentage(% by mole) constituted by the unit(s) derived from the hexamethylenediisocyanate with respect to all the units derived from thepolyisocyanate in the polyurethane polymer is 10% by mole or more and90% by mole or less, the ratio of the percentage (% by mole) of urethanebonds in the polyurethane polymer to the percentage (% by mole) of ureabonds in the polyurethane polymer is 85.0/15.0 or more and 100.0/0 orless, and the diol having an acid group is at least one selected fromdimethylolpropionic acid and dimethylolbutanoic acid, and the acid valueof the polyurethane polymer resulting from the unit(s) derived from thediol having an acid group is 40 mgKOH/g or more and 140 mgKOH/g or less.2. The ink jet ink according to claim 1, wherein the polyurethanepolymer is a cross-linked polyurethane polymer.
 3. The ink jet inkaccording to claim 1, wherein the polyether polyol having no acid groupcontains poly(propylene glycol).
 4. The ink jet ink according to claim1, wherein the ratio of the polyurethane polymer content (% by mass) ofthe ink to the pigment content (% by mass) based on the total mass ofthe ink is 0.05 or more and 2.00 or less.
 5. An ink cartridge,comprising an ink storage portion configured to store an ink, whereinthe ink is an ink jet ink according to claim
 1. 6. An ink jet recordingmethod, comprising ejecting an ink by an ink jet method, wherein the inkis an ink jet ink according to claim
 1. 7. An ink jet recording method,comprising: ejecting an ink containing a pigment and a polyurethanepolymer through an ink jet recording head onto a recording medium; andapplying a liquid composition to the recording medium such that theliquid composition at least partly overlaps the ink, the liquidcomposition destabilizing the dispersion of the pigment in the ink anddecreasing the solubility of the polyurethane polymer, wherein the inkis an ink jet ink according to claim 1.